The joint sponsor still needs to file late and settle any taxes for 2024, and have the IRS tax transcript to be a joint sponsor.
So even if you submit the I-129F before April 15, this year before the 2025 tax return is filed, the time to NOA2 is currently around 10 months, which is still enough time to file the 2025 tax return and download the tax transcript. So by K1 beneficiary interview time, the K1 petitioner and joint sponsor should include the 2023, 2024, 2025, tax transcripts as part of the I-134.